Most power semiconductor devices like silicon-controlled rectifiers (“SCRs”), power transistors, insulated gate-bipolar transistors (“IGBTs”), metal-oxide-semiconductor field effect transistors (“MOSFETs”), power rectifiers, power regulators, or combinations thereof, are assembled in packages that fail to provide sufficient electrical isolation. That is, a metal tab, which typically forms the backside of the packaged device, is electrically coupled, e.g. soldered, to the semiconductor die within the packaged device. This places the backside of the package at the same electrical potential as the semiconductor die.
Power semiconductor devices are designed to operate at relatively high voltages, typically 30 V to 1600 V, or higher, compared to other electronic semiconductor devices, such as logic or memory devices. In a conventional packaged power semiconductor device such as that illustrated in FIG. 1A and FIG. 1B, the backside of the package may be subject to these voltages under normal operation or if a failure occurs. Additionally, the power semiconductor device may be exposed to voltages outside of the intended range during operation, which may electrically couple to the backside of the package.
The high voltages present at the backside of conventional packaged power semiconductor devices may damage other circuit components or may present a safety hazard to operating personnel or to operating equipment built with these devices. Voltages as low as 40 V can be a hazard to operators working with such equipment. Therefore insulating pads or washers are typically used to electrically isolate the backside of the power semiconductor device from the rest of the circuit. In a typical application, the power semiconductor is mounted on a heatsink that is part of an electrical chassis at ground potential.
FIG. 1A illustrates a known semiconductor device providing insufficient electrical isolation. A packaged power semiconductor device 10, an insulating pad 12, a heatsink 14, and a screw 16 are provided. The screw is used to attach the semiconductor device 10 and the insulating pad 12 to the heatsink 14. The semiconductor die (not shown) is attached to a metal tab 21, leads 18 are electrically coupled to the terminal of the die, and the assembly is then encapsulated with encapsulating material 20. The encapsulating material is typically epoxy, plastic, rubber, silicone, or similar materials and is molded, cast, or otherwise formed over the die and related structures.
The heatsink 14 is usually made of a thermally conductive material such as metal, and the insulating pad 12 is typically made of an insulting material such as silicone rubber, mica, or ceramic, and may be in the form of a washer or other shape, rather than a pad. It is desirable that the insulating pad 12 provide electronic isolation between the backside 22 of the power semiconductor device 10 and the heatsink 14 while also providing good thermal coupling to the heat sink. Further improvement of thermal coupling between the semiconductor device 10, the insulating pad 12, and the heatsink 14 can be realized by applying thermal grease or phase change material between the semiconductor device 10 and the insulating pad 12 and/or between the insulating pad 12 and the heatsink 14.
FIG. 1B is a simplified view of the power semiconductor device 10 and the insulating pad 12 mounted to the heatsink 14 with the screw 16. The screw 16 can be made of an insulating material, such as nylon, or additional insulating washers and/or sleeves can be used to isolate the power semiconductor device from the heatsink. In some applications, it may be necessary to provide enhanced heatsinking. Soldering the power semiconductor device 10 to the heatsink 14 will provide superior thermal coupling. However, the heatsink 14 would typically need to be isolated from the rest of the circuit or chassis, and may require a shield to prevent a technician from inadvertently touching an electrically “hot” heatsink, which could be at a lethal voltage.
An advancement of this conventional packaged power semiconductor device is the electrically isolated power semiconductor package as described in U.S. Pat. No. 6,404,065, U.S. Pat. No. 6,534,343, U.S. Pat. No. 6,583,505, and U.S. Pat. No. 6,710,463, all of which are assigned to IXYS Corp. of Milpitas, Calif., and all of which are incorporated by reference herein in their entirety.
A known semiconductor device is disclosed by Noda et al. in U.S. Pat. No. 5,767,573. Noda discloses an intelligent power module (IPM) in which a power device and a control device are soldered onto independent lead frames. The lead frames may be attached to a metal heat sink via an insulative adhesive/solder layer, a circuit pattern layer, and an insulating layer. The components may then be encased in a molding resin, where a bottom surface of the metal heat sink is exposed.
In practice, a need has been recognized for attaching electrically isolated power semiconductor packages to objects, such as heat sinks, via the use of packages configured for widely accepted industry standards. For example, in accordance with some industry standards, packaged power semiconductor devices are mounted via a screw.
A need has also been recognized for increasing the thermal conductivity between power semiconductor packages and objects to which they are attached. For example, it is desirable to increase the thermal conductivity between power semiconductor packages that are mounted to heat sinks via a screw.
Further, a growing diversity of the range of applications for semiconductors has also increased the variations of processing and packaging. This manifests in a large number of different modules, discrete components, and integrated circuit (IC) packages that vary enormously in terms of construction, mounting and contacting structures. The construction offers numerous differences like: molded parts or plastic housings with different potting materials; base plates of copper or aluminum with or without ceramics; and structures for isolating or not isolating the base plates. The mounting and contacting structures include screws, clips, and solder. This variety provides an opportunity to find, for every application, the optimized solution in terms of easy handling (e.g. pick and place for boards), thermal management, sizing and isolation if needed.
To improve existing standard packages, it is important to maintain the package layout (e.g., footprint and size) but to extend the performance characteristics of the package.
Therefore, it is desirable to mount packaged power semiconductor devices having superior electrical isolation and thermal coupling characteristics using packages configured for industry standards. It is further desirable to mount packaged power semiconductor devices using a screw or equivalent attaching structure. It is still further desirable to increase the performance characteristics of a semiconductor device having superior electrical isolation and thermal coupling characteristics while providing the device with an industry-standard layout.